1. Field of the Invention
The present invention relates to a zinc alloy powder for use in an alkaline cell and a method for the production of the same. More particularly, the present invention is related to a non-amalgamated zinc alloy powder for use in an alkaline cell, which has an iron content of 1 ppm or less and contains specific elements added so as to suppress the evolution of hydrogen gas and to improve the leaktightness of a cell, and a method for the production of the same.
2. Prior Art
The mercury contained in an amalgamated zinc powder used as an anode active material in an alkaline cell has been known to be an essential component for such an active material from the viewpoint of suppressing the evolution of hydrogen gas due to the corrosion of zinc and preventing a liquid from leaking from the cell as a result of the evolution of hydrogen gas.
In light of environmental protection, however, a reduction in the mercury content is required in this field. In line with this requirement, it has become possible to suppress the evolution of hydrogen gas through the addition of not only lead but also aluminum, bismuth, indium and the like as additional elements to zinc so that the mercury content is remarkably reduced from 10% by weight to about 1% by weight.
As further social needs, in recent years, it is required to decrease the mercury content of the anode active material to 0% by weight, in other words, to effect non-amalgamation. This non-amalgamation greatly changes the situation, and it has been difficult to decrease the evolution of hydrogen gas to a desired level even when the above-described additional elements are added. That is, although zinc alloy powders as an anode active material having various types of elements added thereto have been proposed (see, for example, Japanese Patent Laid-Open Nos. 22984/1990 and 153950/1986, it has been impossible to attain the desired suppression of hydrogen gas evolution when the mercury content is 0% by weight, though such suppression has been possible even when the mercury content is 1% by weight or less.
Meanwhile, attempts have been made to suppress the evolution of hydrogen gas and to improve the discharge performance by reducing the impurity content of zinc. For example, Japanese Patent Laid-Open No. 123653/1987 describes a reduction in the content of impurities such as iron and chromium. Table 1 on page 4 of the published specification shows that an improvement in the discharge performance while suppressing the evolution of hydrogen gas is attained by reducing the iron content to about 10 ppm in an anode active material which comprises an amalgamated zinc alloy powder containing predetermined amounts of lead, indium and aluminum and containing 1% by weight of mercury.
However, a zinc alloy powder having a mercury content of 0% by weight could not attain the desired effect of suppressing the evolution of hydrogen gas even when the impurity content was reduced to about 10 ppm and additional elements such as lead were incorporated.
Thus the non-amalgamation of an anode active material is accompanied by a difficulty which is fundamentally different from that encountered in the low amalgamation leading to a mercury content of 0.6 to 1% by weight, and there has not been developed any alkaline cell wherein a non-amalgamated zinc alloy powder is used as an anode active material, the evolution of the hydrogen gas is suppressed, and the leaktightness is improved.